Acyloin condensations | Chemistry Optional Notes for UPSC PDF Download

 Page 1


1| 
1.  Acyloin Condensation 
Principle 
The carboxylic acid esters undergoes bimolecular reductive coupling upon refluxing with 
aprotic solvents such as ether, benzene, toluene or xylene to afford a-hydroxy ketone is 
known as acyloin condensation. Symmetrical a-hydroxy ketones (aliphatic analogs of 
benzoins) are commonly known as acyloin; the name is derived by adding the suffix            
‘-oin’ to the name of corresponding acid. 
 The reaction is more favored when R is an alkyl group. With longer alkyl chains, 
higher boiling solvents can be used. Di-esters are used to prepare cyclic acyloins. When 
the acyloin condensation is carried out in the presence of chlorotrimethylsilane, the 
enediolate intermediate is trapped as bis-silyl derivative which is hydrolysed in acidic 
condition to the acyloin. Reaction occurs between two moles of ester (intermolecular 
condensation) or one mole of di-ester (intramolecular condensation). Rearrangement is 
promoted by either acid or base; the thermal acyloin rearrangement can be accelerated by 
high pressure. 
General Reaction 
R
C
O
O
H
2
C
CH
3
2
Na, Xylene
?
R'
C
O
C
O R
R'
C
ONa
C
ONa R
CH
3
COOH
R
C
OH
C
R'
O
H
Ester
1,2-diketone
1
2
Sodium salt of enediol
Acyloin
(a-hydroxy ketone )
2Na
 
Mechanism 
Step 1: Reaction proceeds through free radical mechanism. 
A reaction occurs in presence of metallic sodium; a direct transfer of electron towards 
carbonyl carbon atom takes place to give an intermediate (I) which rapidly dimerize to 
produce unstable intermediate product (II). Rapid loss of both alkoxy groups from 
intermediate (II) gives 1,2-diketone. 
R
C
O
O
H
2
C
CH
3
R'
C
O
H
2
C
CH
3
R
C
OC
2
H
5
ONa
C
OC
2
H
5
ONa R'
R
C
OC
2
H
5
ONa
C
OC
2
H
5
ONa R'
Radical 
dimerisation
Na
Na
O
(I)
(II)
+
 
Page 2


1| 
1.  Acyloin Condensation 
Principle 
The carboxylic acid esters undergoes bimolecular reductive coupling upon refluxing with 
aprotic solvents such as ether, benzene, toluene or xylene to afford a-hydroxy ketone is 
known as acyloin condensation. Symmetrical a-hydroxy ketones (aliphatic analogs of 
benzoins) are commonly known as acyloin; the name is derived by adding the suffix            
‘-oin’ to the name of corresponding acid. 
 The reaction is more favored when R is an alkyl group. With longer alkyl chains, 
higher boiling solvents can be used. Di-esters are used to prepare cyclic acyloins. When 
the acyloin condensation is carried out in the presence of chlorotrimethylsilane, the 
enediolate intermediate is trapped as bis-silyl derivative which is hydrolysed in acidic 
condition to the acyloin. Reaction occurs between two moles of ester (intermolecular 
condensation) or one mole of di-ester (intramolecular condensation). Rearrangement is 
promoted by either acid or base; the thermal acyloin rearrangement can be accelerated by 
high pressure. 
General Reaction 
R
C
O
O
H
2
C
CH
3
2
Na, Xylene
?
R'
C
O
C
O R
R'
C
ONa
C
ONa R
CH
3
COOH
R
C
OH
C
R'
O
H
Ester
1,2-diketone
1
2
Sodium salt of enediol
Acyloin
(a-hydroxy ketone )
2Na
 
Mechanism 
Step 1: Reaction proceeds through free radical mechanism. 
A reaction occurs in presence of metallic sodium; a direct transfer of electron towards 
carbonyl carbon atom takes place to give an intermediate (I) which rapidly dimerize to 
produce unstable intermediate product (II). Rapid loss of both alkoxy groups from 
intermediate (II) gives 1,2-diketone. 
R
C
O
O
H
2
C
CH
3
R'
C
O
H
2
C
CH
3
R
C
OC
2
H
5
ONa
C
OC
2
H
5
ONa R'
R
C
OC
2
H
5
ONa
C
OC
2
H
5
ONa R'
Radical 
dimerisation
Na
Na
O
(I)
(II)
+
 
2| Organic Name Reactions: Principles, Mechanisms and Applications 
 
Step 2: 1,2-diketone is highly reactive, undergoes reduction with metallic sodium to give 
sodium salt of enediol. Finally addition of carboxylic acid affords 1,2-diol which after 
tautomerization resulted into the stable product acyloin ( a-ketol or a-hydroxy ketone). 
 
Applications 
Intramolecular acyloin condensations of diesters have been widely used for synthesis of 
medium and large ring compounds with better yield. 
a) Preparation of cyclic acyloins. 
 
b) Preparation of catenane.